Pluto may have long since said goodbye to its only Earthly visitor, the New Horizons probe, but the information gathered during that brief flyby is still turning up new surprises. NASA scientists have analyzed information sent back from the spacecraft and, comparing it with observations made here on Earth at the same time, found that Pluto is unexpectedly emitting X-ray signals and seems to be "spray-painting" its largest moon, Charon, a vibrant red.

NASA's New Horizons probe made its closest flyby of the dwarf planet in July 2015, before making a few course corrections to get it on track to make a 2019 rendezvous with a planetoid called 2014 MU69, about a billion miles beyond Pluto. Due to the distances involved and the instruments onboard, the data and images collected by New Horizons have been slowly and steadily returning to Earth since that day, and as scientists sift through it, new discoveries are still being made after over a year.

Unexpected X-rays

As a cold, rocky planet that doesn't have a magnetic field, Pluto itself shouldn't be able to emit X-rays – and yet, the Chandra X-ray Observatory has found that it does. Judging by observations made in other parts of the solar system, the signals could be the product of charged particles given off by the sun – the solar wind – interacting with Pluto's atmosphere, but again, there are pieces of the puzzle missing. While the planet is releasing enough gas from its atmosphere to potentially create X-rays, the solar winds aren't strong enough that far away from the sun to account for the strength of the X-rays detected.

"Before our observations, scientists thought it was highly unlikely that we'd detect X-rays from Pluto, causing a strong debate as to whether Chandra should observe it at all," says Scott Wolk, co-author of the study. "Prior to Pluto, the most distant solar system body with detected X-ray emission was Saturn's rings and disk."

One goal of the New Horizons mission was to study the solar winds in the outer solar system, and using an instrument dubbed Solar Winds Around Pluto (SWAP), the probe observed that Pluto interacts with the solar wind in a way similar to Mars, albeit at a lower intensity. Meanwhile, Chandra was trained on Pluto four times in 2014 and 2015, and the X-ray signals it detected are much stronger than expected, based on New Horizons' figures.

According to the team, there are a few possible explanations. The cloud of gases that are trailing behind Pluto could be bigger than SWAP's observations suggest, or the interplanetary magnetic field could be focusing more of the solar wind towards Pluto than would be expected. A ring of neutral gas, formed due to the low density of the solar wind in the region, could also be responsible.

Chandra will continue to search for X-ray signals from other objects near New Horizons' path, which the probe could investigate as it passes. The research was published in the journal Icarus.

Red-headed Charon

X-rays aren't the only puzzle surrounding Pluto's atmosphere. After a year studying the data and images sent back from New Horizons, the mission scientists think they've solved the mystery of the red-tinged polar region on Pluto's largest moon, Charon. Methane is escaping from Pluto's atmosphere and is captured by Charon's gravity, where it settles in the icy regions at the poles. The red color is created when that methane is exposed to sunlight and molecules called tholins are formed, much like the methane snow-capped mountains seen on Pluto itself.

"Who would have thought that Pluto is a graffiti artist, spray-painting its companion with a reddish stain that covers an area the size of New Mexico?" says Will Grundy, lead author of the paper. "Every time we explore, we find surprises. Nature is amazingly inventive in using the basic laws of physics and chemistry to create spectacular landscapes."

The theory of how the red cap got there had been proposed by the mission scientists earlier, but they didn't have a model to support it they used New Horizons' data, specifically the weather at Charon's poles, to form one. In those parts of the moon, the "seasons" tend to be extremely long, with the area cycling through 100 years of nonstop sunlight, before fading into a century-long night. With the temperature dropping to a crisp -430° F (-257° C) during those deep dark winters, methane gas will freeze into a solid.

"The methane molecules bounce around on Charon's surface until they either escape back into space or land on the cold pole, where they freeze solid, forming a thin coating of methane ice that lasts until sunlight comes back in the spring," says Grundy.

When the sun finally begins to warm up the poles again, that methane ice evaporates, leaving behind heavier hydrocarbons on the surface, and it's these that are turned into tholins in the sunlight.

"This study solves one of the greatest mysteries we found on Charon, Pluto's giant moon," says Alan Stern, a co-author of the study. "And it opens up the possibility that other small planets in the Kuiper Belt with moons may create similar, or even more extensive 'atmospheric transfer' features on their moons."